1. Field of the Invention
The present invention relates to a lens apparatus and an image pickup apparatus including the same, and particularly to, for instance, a single focal lens apparatus and a silver-halide film camera which are suitable for a monitoring camera, a vehicle-mounted camera, a video camera, a digital still camera or a silver-halide film camera and on which a concentric-shaped dome cover can be detachably mounted, and an image pickup apparatus including the same.
2. Description of the Related Art
Conventionally, single focal wide lenses that support increase in the number of pixels of an image pickup element and have high optical performance have been proposed as image pickup apparatuses, such as a monitoring camera and a vehicle-mounted camera. For instance, as disclosed in Japanese Patent No. 4186560 and Japanese Patent No. 5045300 and Japanese Patent Application Laid-Open No. 2012-18422, retro-focus single focal wide lenses including a negative or positive first lens unit, a stop and a positive second lens unit have been known.
In many monitoring cameras, a concentric-shaped dome cover is mounted on an image pickup system and used for protection and thermal insulation of the image pickup system. Since the dome cover has a refractive power, optical performance varies according to presence or absence of the dome cover. Japanese Patent Application Laid-Open No. 2011-81110 discloses a technique that corrects variation in optical performance according to presence or absence of a dome cover using a correction lens arranged between the dome cover and an image pickup system.
In recent years, image pickup apparatuses, such as a monitoring camera and a vehicle-mounted camera, are required to include a single focal lens having a wide angle of field.
Unfortunately, as with Japanese Patent No. 4186560 and Japanese Patent No. 5045300 and Japanese Patent Application Laid-Open No. 2012-18422, an image through a lens achieving a wide angle of field where a negative distortion is intentionally retained to be about −80% or stronger has a distorted periphery, which is unnatural.
As image processing techniques have advanced, image pickup apparatuses that electronically perform distortion correction to suppress distortion in an image have been proposed. Unfortunately, electronic enlargement of a compressed image degrades resolution at an angle of field at the periphery.
A design that has a wide angle of field and suppresses negative distortion requires reduction in focal length, which however increases the refractive power of each lens. Accordingly, it is difficult to correct various aberrations.
In recent years, as monitoring cameras, products are required that are image pickup apparatuses including dome covers for protection and thermal insulation of image pickup systems and achieving both high optical performance and reduction in size. Unfortunately, reduction in size of the dome cover increases the refractive power of the dome cover. Accordingly, variation in optical performance increases according to presence or absence of the dome cover. In particular, in a wide-angle lens, variation of a focus position at the periphery of an imaging range increases with respect to the center of the imaging range according to presence or absence of a dome cover. The periphery of the imaging range is thus out of focus with respect to the center of the imaging range. In Japanese Patent Application Laid-Open No. 2011-81110, a correction lens is arranged between a dome cover and an image pickup system to correct degradation of the optical performance. This arrangement increases the interval between the dome cover and the image pickup system. Particularly, in a wide-angle lens, a correction lens tends to be large in size, and the interval between the dome cover and the image pickup system increases, thereby in turn increasing the size of the image pickup apparatus.